This invention relates to an FM stereo receiver, especially noise reduction circuits of the FM stereo receiver for cars.
A conventional FM noise reduction circuit, as shown in FIG. 2, includes an intermediate frequency amplification circuit 1 (IF amplifier, hereafter), including an intermediate frequency input 18, the level detection circuit 9 (level detector) to rectify IF signals from the IF amplifier 1 and output the rectified IF signals into the low pass filter 10 (LPF) and high pass filter 31 (HPF), the muting circuit 4 to attenuate a stereo composite signal demodulated by the FM detection circuit 2 and output by noise control (N.C.) circuit 3 depending on the output signal (roughly the direct current only) of LPF 10, the rectification circuit 33 to rectify the output (composed of the alternating current only) of HPF 31, LPF 35 to smooth the output of rectification circuit 33, the adder 17 to subtract the output of LPF 35 from the output of LPF 10, the high cut control circuit 6 (HCC circuit) to attenuate the high frequency components of MAIN signal (L+R) in the composite signal and the stereo noise control circuit 7 (SNC circuit) to control the stereo blend quantity by attenuating the SUB signal (L-R) in the same composite signal controlled respectively by the output of the adder 17, and the multiplex demodulation circuit 8 (MPX circuit) to demodulate the stereo audio signal L and R from the composite signal processed respectively by going through the above muting circuit 4, HCC circuit 6, and SNC circuit 7. Left and right output channels 18, 19 output the demodulated stereo audio signal.
These operations are broadly divided into suppression of noise increment in the stereo demodulation output by the FM demodulation noise with the input signal lowering under the weak electric field and reduction of a multi-pass distortion and multi-pass noise by a mutual interference of the direct wave and reflecting wave. First of all, as for the former, in the weak electric field the C/N ratio decreases at the high frequency amplification circuit input, because of the lowering of the input signal level from the antenna. The FM demodulation noise voltage, therefore, increases at the output of FM detection circuit 2 in inverse proportion to the antenna input level. When the IF input signal goes down to or below the set threshold value, the SNC circuit 7 controlled by the first direct current voltage of the output of LPF 10 in proportion to the IF input signal level (i.e., in proportion to the antenna input level) attenuates the SUB signal frequency band on the composite signal and suppresses the noise increment on the demodulation stereo signal. In addition, when the IF input level goes down, the high frequency component of MAIN signal (L+R) is attenuated by the HCC circuit 6, moreover the noise increment is suppressed. With the operation of these HCC circuit 6 and SNC circuit 7 the S/N ratio of demodulation audio signal in the weak electric field is ensured till the limit. And when it turns to the weak electric field, the IF signal input level goes down and is below the limiting sensitivity of IF amplifier, the FM demodulation noise radically increases and it is very offensive to the ear. To solve this problem, all FM demodulation output signals including L+R MAIN signal are attenuated by the muting circuit 4.
Next, it describes about the latter, the reduction of multi-pass distortion and multi-pass noise. When there is a multi-pass, the multi-pass distortion or the multi-pass noise occurs to the demodulated output by the interference of a direct wave and indirect wave. This noise of distortion remarkably appears in the high frequency component of the FM demodulation output, L-R SUB signal band and L+R MAIN signal high frequency, so that, when there is the multi-pass, the multi-pass noise and distortion appeared in the demodulated stereo signal can be reduced by the process with the SNC circuit 7 and the HCC circuit 6, which process is performed by the following interference detection means. When there is the multi-pass and that frequency is modulated (FM), the instant relative phases of reflecting wave and direct wave are changed by the instantaneous value of modulation signal, so the AM modulation occurs by the mutual interference to the total antenna input signals of the reflecting wave and the direct wave. Therefore, the alternating current component is multiplied to the output of level detector 9, so the second direct current voltage will be obtained by abstracting it with the HPF 31, rectifying it with the all wave rectifier 33, and smoothing it with the LPF 35. Then adder 17 subtracts the second direct current voltage from the first direct current voltage, and the output of adder 17 controls and the HCC and SNC circuits. In other words, the HCC and SNC circuits are controlled as same as the weak electric field when there is the multi-pass.
The FM noise reduction circuit, as described above, is essential to the FM receiver for cars when the electric field strength is frequently changed by influences of buildings and so forth. With a mobile receiver, multi-pass interference frequently occurs.
Now in the conventional FM noise reduction circuit, when the multi-pass interference is extremely serious, that is, the level between direct wave and reflecting wave is close and the delay time is comparatively long, the distortion or noise reached to the reduction of L+R MAIN signal and can not be processed by the HCC and SNC circuits, the far bigger noise is replaced than the demodulation signal by the usual modulation, and it is extremely offensive to the ear.
More, in the conventional technique (NEC electric device, the reference No. IP-8019, November, 1989) as well, the HCC circuit and SNC circuit are only offered for the reduction of multi-pass noise and distortion, and when the multi-pass interference is extremely serious the far bigger noise is replaced than the demodulation signal, and it is inconvenient.
Further, there is the technique as mentioned in ICs for electrically tuned car radios (August, 1989, pp 1-20, Phillips Components) other than that, but when the multi-pass interference is extremely serious the far bigger noise is replaced than the demodulation signal, and it is inconvenient.